This description relates to pressure-related feedback instability mitigation, for example, in an active noise reduction system.
The presence of ambient acoustic noise in an environment can have a wide range of effects on human hearing. Some examples of ambient noise, such as engine noise in the cabin of a jet airliner, can cause minor annoyance to a passenger. Other examples of ambient noise, such as a jackhammer on a construction site can cause permanent hearing loss. Techniques for the reduction of ambient acoustic noise are an active area of research, providing benefits such as more pleasurable hearing experiences and avoidance of hearing losses.
Some noise reduction systems utilize active noise reduction techniques to reduce the amount of noise that is perceived by a user. Active noise reduction (ANR) systems can be implemented using feedback approaches. Feedback based ANR systems typically measure a noise sound wave, possibly combined with other sound waves, near an area where noise reduction is desired (e.g., in an acoustic cavity such as an ear cavity). In general, the measured signals are used to generate an “anti-noise signal,” which is a phase inverted and scaled version of the measured noise. The anti-noise signal is provided to a noise cancellation driver, which transduces the signal into a sound wave that is presented to the user. When the anti-noise sound wave produced by the noise cancellation driver combines in the acoustic cavity with the noise sound wave, the two sound waves cancel one another due to destructive interference. The result is a reduction in the noise level perceived by the user in the area where noise reduction is desired.
Feedback systems generally have the potential of being unstable and producing instability based distortion. In feedback systems, the input to a system being controlled (called the “plant”) is provided by forming a feedback loop that compares the output of the plant to a desired input or reference signal. One or more compensators within the feedback loop provide gain over a particular frequency spectrum to drive the difference between the output and desired input near zero over that frequency spectrum. Instability may result if the gain of a feedback loop is greater than 1 at a frequency where the phase of the feedback loop is 180°.